The present invention relates to the filtration of effluents by means of a baghouse and, in particular, to a method and apparatus for the effective control of the precoating of the bags of the baghouse.
As a result of the contemporary emphasis on environmental quality, various air pollution control means have been developed. One such means for controlling air pollution to which this invention particularly relates has been the separation of unwanted particulate matter from a gas stream by fabric filtration.
The dust collection apparatus used to accomplish such particle separation is known in the trade as a "baghouse". Basically, a baghouse is a sheet metal housing divided into at least two chambers by a tube sheet having a plurality of vertically suspended fabric tubes or bags in which cylindrical wire cages may be provided for skeletal support. A plurality of such chamber pairs may be combined to comprise a baghouse with several distinct parallel filtration compartments. A particle-laden gas stream, induced by the action of a fan, flows into the lower chamber (dirty-air chamber) of a compartment wherein dust accumulates on the cloth bags as the gas passes through the fabric into the upper chamber (clean-air chamber) and out the exhaust. Continuous operation of the unit causes a cake of dust to build up on either the inside or the outside of the bags, depending on which side of the bag is exposed to the inlet of the effluent. Such a dust cake will, unless removed, reduce and eventually stop the flow of gas through the filter. In order to alleviate the direct contact of the dust upon the bags themselves and the resultant complications to the system, the bags are usually precoated with a suitable precoat material such as limestone powder.
It will be easily appreciated that the most critical and yet sensitive elements of such a filtration system are the bags themselves. Due to the criticality of the function the bags perform the precoating of the bags is necessary for several interrelated reasons. First, the precoating of the bags prevents a solid coating of dust matter from accumulating directly on the bags. The deposition of such a solid coating on the bags is harmful in various ways. For example, a solid coating on the bags results in a higher resistance to flow through the system and a resultant drop in overall system efficiency. Also, the direct contact of particulate matter in the form of a solid coating on the fabric bags causes both direct damage to the bags and to the system as a whole. The direct contact of particulate matter on the costly filtration bags greatly accelerates the deterioration of the fabric from a variety of causes including chemical attack, thermal erosion, abrasion and, mechanical stress through repeated flexing. Should bag failure occur during operation, the results would include a sudden increase of contaminants in the outlet stream, often in excess of legal and/or acceptable limits. Also, as the replacement of single bags is not practical, the operating personnel usually wait until several bags have failed and the contaminant content in the outlet stream has increased above acceptable limits before the entire baghouse system is shut down and the damaged bags are replaced.
A second reason that the bags are precoated prior to commencement of the filtration process is that precoating aids in the removal of particles from the effluent stream. Filtration bags treated with current precoat materials are more effective in the removal of particulate matter from a stream than untreated bags because the precoat material provides increased filtration surface area and improved chemical attraction to the particulates. In addition, the precoat material retains an adequate degree of porosity through the filter to avoid excessive flow resistance therethrough. Yet another reason for the application of a precoat material to baghouse bags is that it facilitates the removal of the filter cake from the bag surface.
From the above, the significance of precoating the filtration bags of a baghouse is readily apparent. As such, the precoating process must be properly controlled to assure complete precoating in order to gain full advantage of the above-described reasons for precoating. However, in many baghouse systems, the precoating procedure is improperly employed causing harm to the baghouse system in general, and to the filtration bags in particular. A widespread problem in baghouse precoating is a complete or partial failure to precoat the bags due to improper precoat control. As a specific example, precoating is often improperly performed due to the failure of the system to transmit the precoat material to the filtration bags. In usual baghouse construction and operation, the precoat material is fed into a precoat duct in communication with the baghouse by a precoat feeder and then drawn onto the baghouse bags by a fan which induces a negative pressure within the baghouse. Typically the fan which draws the precoat material through the precoat duct is the same fan which draws the effluent to be filtered from its source. However, conventional baghouse system design does not employ a fan which is of sufficient capacity to draw precoat material through the precoat duct while simultaneously drawing effluent to the baghouse from its source. The problem which arises frequently in such a situation is that the precoat material is not drawn through the precoat duct, accumulates therein and clogs the duct. Of course, when the precoat duct is clogged, precoat material cannot pass therethrough. This causes the bags to fail to be precoated and all of the above discussed problems inherent with non-precoated bags evolve. Further, the unclogging of the precoat duct involves an extensive, expensive clean-up operation and probable system downtime.
One possible solution to the problem of precoat duct clogging due to insufficient suction to draw precoat material therethrough would be to install a separate precoat duct blower to assist precoat material flow. However, this attempted solution would increase capital costs for the system as well as operating and maintenance costs. Another possible solution is to add costly precoat flow aids to the precoat material which would increase operating costs and are often ineffective in assuring proper precoat material transmission.
The subject invention is directed toward an improved means for controlling the precoating of baghouse filtration systems which overcomes, among others, the above discussed problems and provides a baghouse precoat control method and means which are effective in the control of baghouse preoating yet do not necessitate a significant increase in baghouse capital, operating and/or maintenance costs.